1. Field of the Invention
The present invention relates to a burst-mode optical receiver and more particularly to a burst-mode optical receiver that can determine an input signal and generate on its own a reset signal for initialization during intervals between the packets.
2. Description of the Related Art
The next generation of communication services require FTTH (Fiber-To-The Home), which installs optical fiber directly into subscribers' homes in order to provide more information to the subscribers at a higher speed. However, it is costly to replace existing copper-based subscriber lines with optical subscriber lines, such as FTTH. In this regard, PONs (Passive Optical Networks) have been proposed for the provision of low-cost optical subscriber lines.
FIG. 1 illustrates a passive optical network system which consists of mainly an optical line termination (OLT) located in a central office, a plurality of 1×N passive optical splitter, and a plurality of optical network units (ONUs) located in the subscriber's premise. In operation, each node transmits data or packets to another node using a predetermined time slot. It is possible for multiple subscribers to transmit data via a single-fiber optic strand through a time-division multiplexing scheme, so that the receiver (OLT) can receive data from each subscriber at any time. Note that the multi-access network is different from a point-to-point link in that it tends to generate a burst-mode data of varying sizes and phases from received data packets due to the optical loss, which occurs via different transmission paths. Further, the size of data packets received tend to vary due to the difference in the path lengths to the subscribers' premises.
Accordingly, the current trend is to use a burst-mode optical receiver capable of receiving data of various sizes and phases and then to restore the data to the same size and phase for all packets. The burst-mode optical receiver extracts a detection threshold as a reference signal for data determination from each burst packet received. The burst-mode optical receiver must have a function of restoring data by amplifying the data symmetrically based on the extracted detection threshold.
Furthermore, in the burst-mode optical receiver, a pre-amplifier together with an optical detector form a front-end of the optical receiver, such that the pre-amplifier can convert an input optical signal to an electrical signal and then amplify the signal with a minimum level of noise. As the pre-amplifier significantly affects the overall receiving sensitivity of the optical receiver and the receiving signal range, it is required to have high gain, broad bandwidth, and low-level noise characteristics. Therefore, the pre-amplifier must have a sufficiently low level of noise kept at a low BER, while ensuring a sufficient output voltage for a low input current. For a high input current, the noise must be low enough to avoid a pulse-width distortion. Accordingly, the pre-amplifier of the burst-mode optical receiver is able to detect the amplitude fluctuations of a signal inputted to an amplifier and automatically control the gain utilizing an automatic gain controller, thereby maintaining the amplitude of an output signal at a constant level.
BiCMOS processes can easily embody an automatic-gain-control (AGC) device, which reduces the effective gain for a larger input signal, thereby improving the overload characteristic. This is mainly in part that in BiCMOS processes, a bipolar junction transistor (BJT) can be used in an amplified segment requiring a high-speed characteristic, and a CMOS element can be used as an active variable resistor that is adjustable by a voltage. The automatic-gain-control device uses the direct current part of an output as a control voltage in order to maintain the gain to be constant. The most commonly used construction feeds back a part of the output voltage to a pre-amplifier in order to control the gain.
The automatic-gain-control device must operate rapidly and respond to be used in a burst-mode operation. The loop time of currently commercially-available automatic gain controllers is typically about 5 μsec; however, to be used in a burst-mode operation, automatic gain controllers must respond more rapidly within tens of nano seconds at the longest.
FIG. 2 illustrates the construction of a burst-mode optical receiver comprising a conventional automatic gain controller. As shown, the burst-mode optical receiver comprises an optical detector 8, a pre-amplifier 10, an automatic gain controller 20, a peak detector 30, and a buffer 40.
The optical detector 8 is configured to convert an input optical signal to a current signal. The pre-amplifier (or trans-impedance amplifier; hereinafter referred to as “TIA”) 10 converts the current signal detected at the optical detector 8 and outputs a corresponding signal to the buffer 40. The peak detector 30 detects a peak value which is the highest level of the signal outputted from the buffer 40, then outputs the peak value to the automatic gain controller 20. The automatic gain controller 20 receives the output from the peak detector 30 and the TIA 10 and generates an AGC signal, which is forwarded to the TIA 10. The peak detector 30 has a hold capacitor therein. When charged, the hold capacitor memorizes an initial AGC signal and continuously maintains the voltage level. Thus, when the hold capacitor is discharged, the AGC signal is terminated.
The automatic gain controller of the prior art as mentioned above can detect a peak value of a high-level signal without difficulty. However, this automatic gain controller has a problem in determining the presence of a low-level signal because it is difficult for the controller to correctly determine the actual voltage-level information only through detecting the peak value of a low-level signal.